1. Field of the Invention
The present invention concerns a display device and it relates to a liquid crystal display device with less image sticking during operation and not causing image defects such as black unevenness even after operation for a long time.
2. Description of Related Art
In a liquid crystal display device, a TFT substrate having pixel electrodes and thin film transistors (TFT) formed in a matrix and a counter substrate opposed to the TFT substrate and having color filters, etc, formed at portions corresponding to the pixel electrodes of the TFT substrate are disposed, and liquid crystals are put between the TFT substrate and the counter substrate. Then, images are formed by controlling the transmittance of a light by liquid crystal molecules on every pixel.
Since the liquid crystal display device is flat and light in weight, the application use thereof has been extended in various fields including large sized display devices such as TV, and cellular phones or DSC (digital still cameras). On the other hand, view angle characteristics become a problem in the liquid crystal display devices. The view angle characteristics concern a phenomenon that luminance or chromaticity changes between a case of observing a screen from the front and a case of observing the screen obliquely. For the view angle characteristics, an IPS (in plane switching) system of operating liquid crystal molecules by an electric field in a horizontal direction has excellent characteristics.
An example of such liquid crystal display devices of the IPS system is disclosed, for example, in Japanese Patent Laid Open No. Hei 09-73101.
The liquid crystal display device is used for a long time. When the liquid crystal display device is used for a long time, it sometimes causes image defects. The image defects can be classified into two types. The first defect is irreversible change of the displayed images after long-time use which includes, for example, those defects referred to as black unevenness in a normally black mode of displaying black in a state of not applying a voltage. The second type defect is a phenomenon that an image remains on a screen when it is displayed for a long time and this is referred to, for example, as DC image retention.
An example of the black unevenness is shown in FIG. 12. In FIG. 12, the black unevenness is depicted as BB. Black variation is a phenomenon that a certain region on the screen is blackened more compared with other regions. The phenomenon may be caused when a liquid crystal display device is operated for a long time, for example, 1,000 hours or more. The phenomenon is considered to be attributable to contamination of liquid crystals with impurities which lowers the insulation resistance of liquid crystals by long-time operation of the liquid crystal display device.
The DC image retention is a phenomenon that a pattern as shown in FIG. 10 remains sickly on the screen in a case where a halftone solid gray pattern is displayed after displaying a pattern as shown in FIG. 10 for a predetermined time. Such a phenomenon may be explained, for example, as that when impurities are deposited to alignment films that put liquid crystals therebetween, the impurities are charged by video signals and image retention remains when the charges are maintained for a predetermined time. Since the DC image retention disappears when the charges of the impurities on the alignment film are eliminated, this is a reversible phenomenon.
The present invention intends to eliminate the black unevenness and the DC image retention as described above.